1. Field of the Invention
The present invention relates to an electro-luminescent display (ELD) including a current mirror for uniform illumination throughout the whole display.
2. Discussion of the Related Art
An ELD, into which electrons and holes are injected, uses the recombination between electrons and holes to generate an electro-luminescence. An ELD is a next generation display technology and has the benefits of not requiring a back light, providing a thinner panel, and achieving reduced power consumption.
In an active ELD, a plurality of pixels are defined by a plurality of gate lines and data lines intersecting each other. In the respective pixels, power supply lines are arranged in the same direction as the data lines. A pixel has at least one switching device such as a thin film transistor (TFT), a storage capacitor, and an electro-luminescence (EL) portion.
When an ELD includes a pair of TFTs in each pixel, an EL exciting signal and a scanning signal are used. The EL portion is selected by a logic TFT and exciting power of the EL portion is controlled by the power of the other TFT. A storage capacitor is provided for maintaining the exciting power of the EL portion in the selected cell.
FIG. 1 is a schematic of an equivalent circuit of an ELD according to a related art. Referring to FIG. 1, a plurality of pixel regions are defined by a plurality of gate lines, for example, G1, G2, and data lines, for example, D1, D2, arranged to intersect each other.
First, TFTs M1 are connected to the intersections of the gate lines and data lines, for example, G1 and D1 in a pixel. A storage capacitor CSTO and a gate of a second TFT M2 are connected in parallel to a source of the first TFT Ml. An electro-luminescent diode EL, which is a light-emitting device, is connected to a source of the second TFT M2. A gate driver (not shown in the drawing) is connected to one stage of the gate lines and supplies each of the gate lines with a proper scanning signal. A data driver (not shown in the drawing) is connected to one stage of the data lines and supplies each of the data lines with a data voltage for driving a corresponding electroluminescent diode EL.
The following description explains the operation of the above-described ELD. After a first gate line G1 has been turned on for selecting a specific pixel, a predetermined voltage from a data signal in the first data line D1 is applied to a node A through the first TFT M1. Thereafter, the first gate line G1 is turned off. Until the first gate line G1 is turned on again, the storage capacitor CSTO maintains the voltage at the node A, while the second TFT M2 functions as a drive switch for supplying the EL diode with a fixed current for emitting light.
In general, the drive switch is driven in the saturation region, and the drive current I depends on the following formula, I=xc2xd XxcexcnCo(W/L)(VGSxe2x88x92VTH)2, where xcexcn is the mobility of an electric field, Co is the capacitance of a gate insulating layer, W is the channel width, L is the channel length, VGS is the voltage at the gate and source electrodes, and VTH is the threshold voltage.
Unfortunately, as the display size of the ELD of the related art increases, the deviation in the threshold voltage VTH between the TFTs in each of the other pixels increases, especially on a large substrate. This occurs because the characteristics of the silicon film that constitute the TFTs are irregular throughout the whole pixel array. Specifically, when TFTs made of polycrystalline silicon are used as the switching devices, the irregularity in threshold voltage between the TFTs gets worse due to the difficulty in providing a polycrystalline film having silicon grains that are uniform throughout the whole surface of the substrate.
Therefore, the threshold voltage VTH of the second TFT differs from the first TFT in each pixel even though the same VGS is applied to the first and second TFTs. Thus, the brightness of the image throughout the display is not uniform as different amounts of current flows through the respective EL diodes that are driven by the switching devices in each of the pixels.
To overcome the problems described above, preferred embodiments of the present invention provide an ELD having uniform brightness throughout the whole display by supplying the respective EL diodes with uniform drive currents with the use of a current mirror, even though the VTH of the switching devices in each pixel is not the same.
A preferred embodiment of the present invention includes a substrate, a gate line on the substrate, a data line crossing the gate line, a first TFT for selecting an arbitrary pixel by a gate signal, wherein a gate of the first TFT is connected to the gate line, a current mirror for outputting a signal to the arbitrary pixel selected by the first TFT by receiving a data signal from the data line at the same time that the current mirror is being voltage driven, the current mirror including a second TFT and a third TFT, and an electro-luminescent diode connected to the current mirror, wherein the diode is driven by a signal output from the current mirror.
In another preferred embodiment of the present invention, a method of manufacturing an ELD includes the steps of providing a substrate, forming a gate line on the substrate, forming a data line that crosses the gate line, forming a first TFT for selecting an arbitrary pixel by a gate signal, and connecting a gate of the first TFT with the gate line, providing a current mirror that receives a data signal from the data line as the current mirror is being voltage driven, and outputs a signal to the arbitrary pixel selected by the first TFT, and providing an electro-luminescent diode for receiving the outputted signal of the current mirror.
In preferred embodiments of the present invention, the voltage is continuously supplied to the current mirror such that the current mirror is being voltage driven at the same time that the current mirror receives the data signal from the data line.
Thus, in the present invention, the ELD has uniform luminescence throughout the whole display despite variations in the VTH of the switching devices since the current that is output from the current mirror to the respective electro-luminescent diode is uniform throughout the whole display.
Other features, elements and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.